1. Technical Field
The present disclosure is generally directed towards the field of acoustic imaging, e.g., photoacoustic imaging, ultrasonic imaging, and the like. More particularly, exemplary embodiments of the present disclosure relate to new and useful methods for the production of high-end transducer arrays, e.g., transducer arrays that may be advantageously employed in acoustic imaging applications. Exemplary embodiments of the present disclosure also relate to capacitive micro-machined ultra sonic transducers (cMUTs) that provide improved uniformity and reliability based on the advantageous production methods disclosed herein.
2. Background Art
In acoustic imaging applications, high-end two-dimensional arrays of ultrasound (US) transducers enable active beam steering/focusing and offer/support real-time three-dimensional imaging applications. Furthermore, the quality of such transducer arrays is a key to overall acoustic imaging product performance and often times is a decisive factor in product differentiation. To this end, methods affecting reliable, efficient, low-cost production of US transducer arrays are in high demand.
Current technology for manufacture of US transducer arrays generally involves one of: (i) techniques for fabrication of piezoelectric micro-machined ultrasonic transducers (pMUTs) that generally involve thin-film processes and application of piezoelectric stack technology, and (ii) techniques for fabrication of capacitive micro-machined ultrasonic transducers (cMUTs) that generally involve a sacrificial release process wherein a cavity is created underneath a membrane by depositing a sacrificial layer on the carrier substrate.
In pMUT manufacture, bonded layers of piezoelectric ceramic, composite and simple polymer matching layers are provided. A diamond saw may be used to dice individual elements. Interconnection may be achieved by a conductive layer on the front surface for common, flex traces to element. Alternatively, pMUTs can be made using lithographic techniques, whereby the need for a diamond saw is obviated. Signals are generally transmitted from the pMUT to an ancillary system through coaxial cable or the like. Single crystal manufacture can provide/support improved performance, but at a cost that may not be competitively viable.
Current methods for the production of cMUTs may involve sacrificial etch processes, whereby a vacuum cavity is created beneath a silicon nitride membrane. For an overview of current cMUT-related processing methods, see “Capacitive Micromachined Ultrasonic Transducers: Fabrication Technology,” A. S. Ergun, et al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 52, (12), December 2005. Wafer bonding techniques for manufacture of cMUTs have also been disclosed.
A schematic cross-section of an exemplary conventional one-dimensional cMUT element 10 is depicted in FIG. 1 (prior art). As depicted therein, a typical cMUT element (un-collapsed) includes (from bottom up) a bottom electrode 12b on a silicon substrate, an insulating layer 14b, a membrane 14a, a top electrode 12a, and an oxide passivation layer 18. Typically, the membrane 14a and insulating layer 14b are configured so as to define a vacuum cavity 16, e.g., a cylindrical cavity, therebetween.
Application of a bias voltage effects a charge which pulls the membrane and substrate closer together. However, if the membrane/substrate are brought too close together, collapse may occur. In traditional production of cMUT arrays, small holes (often times in the sub-100 nm order) and channels are etched through the membrane layer to produce the depicted structure, although larger etched channels (on the order of 2-5 microns) have been disclosed. Relatively expensive and complicated equipment/techniques may be needed to pattern the holes in the membrane for the sacrificial etch, e.g., high-resolution e-beam lithography.
Conventional cMUTs (uncollapsed) carry with them inherent disadvantages, including non-linear behaviour, narrow operating range, low capacitance, and high sensitivity to manufacturing variability. In addition, a relatively high bias voltage is required. It is noted that, for an un-collapsed cMUT, the force on the membrane is proportional to the square of the charge. Thus, increasing the force decreases separation which, in turn, increases capacitance, and (with a voltage bias) increases the charge. Consequently, the membrane will eventually collapse when positive feedback overcomes the rigidity of the membrane; the result of this collapse is formation of a collapsed cMUT.
Collapsed cMUTs have been investigated. For example, collapsed cMUTs have been formed wherein the membrane is collapsed to the bottom of the cavity by applying a DC bias voltage. Collapsed cMUTs can offer improved performance. However, problems specific to collapsed cMUTs have yet to be adequately resolved and/or addressed, e.g., manufacturing-related problems, issues related to array uniformity, and overall reliability. For example, the relatively high voltage needed to produce membrane collapse creates significant complications, e.g., increased cMUT sensitivity to charging. Such relatively high voltages may also negatively impact on associated electronics, e.g., chips.
Thus, a need remains for effective and reliable manufacturing methods for fabrication of collapsed cMUTs that exhibit desirable performance properties on a consistent and cost-effective basis. As will be apparent to persons skilled in the art from the disclosure which follows, the advantageous methods disclosed herein meet this unresolved need and support commercial manufacture of collapsed cMUTs, with the attendant benefits associated therewith.